The efficiency of operation of a cryogenic dynamoelectric machine with a superconducting field winding is determined by the current-carrying capacity of the winding. The current-carrying capacity of the winding is dependent upon the quality thereof in many respects.
The prior art discloses a rotor of a cryogenic dynamoelectric machine wherein a superconducting field winding comprises individual flat coils arranged one above the other and separated by plates (see, e.g., U.S. Pat. No. 3,991,333). The axis of the coils is normal to the axis of the rotor of this machine. Rigid retaining of the winding is provided by bolts equally spaced along the length of the rotor. Such a retaining construction of a field winding is easy to fabricate; however the construction also has a disadvantage in that nonuniform cooling of the winding may be present and end portions of the winding may be cooled less than slot portions thereof. In addition to this disadvantage of the known rotor, other problems are present. First, it is impossible to attain a uniform compression of the coils around the rotor circumference, since compressive forces of a shroud are directed along the axis of the rotor and the side surfaces of the coils are at an angle to the direction of these compressive forces. Second, the shroud holds together the entire rotor assembly without providing additional compression of the coils of the superconducting field winding.
Also known in the art is the rotor of a cryogenic dynamoelectric machine comprising a superconducting field winding made of coils placed in radial slots of a supporting structure and held therein by wedges (see, e.g., U.S. Pat. No. 3,891,875). The radial slots of the supporting structure are separated by teeth. The side walls of each slot are provided with axial grooves in which the wedges are fitted. The wedges support the winding coils through underwedge liners and thus securely retain the coils in the slots. The supporting structure is embraced by an annular shroud fitted with a radial interference on the outer cylindrical surfaces of the teeth and on the end portions of the winding so that a radial clearance is formed between the shroud and the surfaces of the wedges, facing the shroud. The shroud provides sealing of a bath with a coolant which cools the field winding.
One disadvantage of this rotor is that on cooling of the rotor, as a result of an appreciable difference between the temperature coefficients of linear expansion of the materials of the winding (copper) and supporting structure (nonmagnetic steel or titanium alloy), a radial dimension of the coils reduces much more than radial dimensions of the wedges and teeth. As this takes place, the originally tight wedging of the coils is loosened, and loosening of the coils fixed in the slots of the supporting structure occurs. This affects the current-carrying capacity of the superconducting field winding, which is determined by rigidity with which the coils are retained in the slots of the supporting structure. A reduction of the current-carrying capacity of the winding, that is, of a critical current thereof, results in a decrease of a winding safety factor for current and hence in a decrease in dynamoelectric machine reliability.